CN108110799B - Virtual synchronous control method and device for HVDC grid-connected island doubly-fed wind farm - Google Patents

Abstract

The invention relates to a virtual synchronization control method and device for high-voltage direct current grid-connected grid-connected island doubly-fed wind farms. The black start of the DC system provides the initial power supply; the VSG is applied to the grid-connected and normal operation control of the DFIG wind turbine. After the VSG realizes the reactive voltage droop control through the rotor-side converter, the grid-side converter can be switched flexibly It is controlled by unit power factor, and the DC power supply is withdrawn at the same time; the frequency adjustment and correction control method is adopted, and the frequency stability and active power balance of the sending end bus of the system are realized through the rectifier; Virtual synchronization control of the network. The invention can reduce power loss and construction cost, increase stability, and can be applied in the field of high-voltage direct current grid connection of new energy.

Description

Virtual synchronous control method and device for HVDC grid-connected island doubly-fed wind farm

technical field

The invention relates to the field of high-voltage direct current grid connection of new energy sources, in particular to a virtual synchronous control method and device for high-voltage direct current grid connection of island doubly-fed wind farms.

Background technique

In recent years, offshore wind farms and large-scale wind farms far from the main grid have emerged. It is particularly noteworthy that, for the geographical locations of these two types of wind farms, there is no existing AC grid to transmit the power generated by them, so they can be called island wind farms. Transferring the power generated by isolated wind farms to the main AC grid is a key technical challenge. First, wind turbines based on traditional vector control can generally be equivalent to a current source, and an external voltage source is required to assist multiple wind turbines to achieve stable operation. Second, the double-fed wind turbine (DFIG) has become one of the typical models in wind farms, and its startup process requires external energy to provide rotor excitation.

High-voltage direct current (HVDC) is widely regarded as an effective and economical solution to the above-mentioned power transmission problems, especially for long-distance large-capacity wind farms. In terms of basic structure, there are currently two typical HVDC technologies: traditional high-voltage direct current (LCC-HVDC) based on phase-controlled converters and flexible high-voltage direct current (VSC-HVDC) based on voltage source converters. Among them, LCC-HVDC has some obvious advantages over VSC-HVDC, such as relatively larger capacity, smaller loss and lower construction cost.

However, under the traditional control strategy, the islanded DFIG-type wind farm cannot be directly incorporated into the LCC-HVDC system. First, the sending-side converter station of LCC-HVDC cannot provide rotor excitation during startup for DFIG, nor can it provide a voltage reference during normal operation for wind farms. Second, the wind turbine based on traditional vector control cannot provide the commutation voltage for the LCC-HVDC sending-end converter station.

To solve this problem, three typical solutions have been proposed in the literature. 1) Coordinated control of DFIG and LCC-HVDC; 2) Additional configuration of STATCOM (Static Synchronous Compensator) on the HVDC sending end bus; 3) Using VSC-HVDC to realize HVDC grid connection of wind farms.

In the first solution, the controller of the wind turbine adjusts the stator flux linkage ψ _s of the DFIG, and the controller of the HVDC rectifier adjusts the system frequency ω _s . From the steady state relationship U _s =ψ _s ω _s , it can be known that the system voltage and frequency are both equal. controllable. However, this scheme has some serious shortcomings or defects: 1) The start-up process of the system and its control strategy are the premise of the normal operation of the system, and must be studied carefully. But in this solution, the black start process of the system is not mentioned. 2) In this scheme, the control method of the DFIG is still implemented based on the vector orientation of the Phase Locked Loop (PLL). It is particularly noteworthy that the connection of DFIG islands to LCC-HVDC is similar to that of passive loads. Under the condition of no strong AC grid support, there is a dynamic coupling between the output of the converter based on PLL vector directional control and the voltage at the common coupling point of the wind farm relationship, which can easily lead to the risk of frequency (synchronization) instability. 3) In this scheme, the vector control method of DFIG makes the active power and frequency approximate decoupling relationship, so the HVDC control of the sending-end bus frequency cannot effectively control the sending-end active power balance, and it is difficult to control the system frequency stability.

In the second scheme, the large-capacity STATCOM is configured on the high-voltage DC sending end bus. The STATCOM can control the voltage amplitude and frequency of the sending-end bus to be constant, play the role of a constant voltage source, and adjust reactive power compensation at the same time. In addition, the DC voltage of the STATCOM can be used as a sign of the active power balance at the sending end, and the rectifier controls the DC voltage of the STATCOM, which can realize the active power balance at the DC sending end. The disadvantages of this scheme are also obvious: 1) The necessary condition for the stable operation of the system is the normal operation of STATCOM. Therefore, the reliability requirements of STATCOM are very high. Once the STATCOM fails, the entire system will face great risk of instability, collapse or even outage. 2) In order to ensure the stability margin of the system, the requirements for the STATCOM capacity and its DC side capacitance are also very high. 3) Once the large-capacity STATCOM is put into use, it must play a real-time adjustment role, which will cause large power loss and operation and maintenance costs.

In the third scheme, in order to realize the black start of the system and the independent controllability of active and reactive power at the sending end, VSC-HVDC is used instead of LCC-HVDC for the HVDC grid connection of wind farms. However, VSC-HVDC is still limited by its small capacity and high cost.

In recent years, virtual synchronization machine technology (VSG) has emerged. Since VSG can simulate the grid-friendly characteristics of synchronous generators, many excellent VSG control algorithms have been proposed and applied in demonstration projects. However, there is still a gap in applying the VSG control idea to the LCC-HVDC grid-connected control strategy of islanded DFIG wind farms.

SUMMARY OF THE INVENTION

In view of the above problems, the purpose of the present invention is to propose a virtual synchronous control method and device for the grid connection of island doubly-fed wind farms with high-voltage direct current, which has higher capacity, lower power loss and construction cost, and can increase the system power consumption. stability.

In order to achieve the above object, the present invention adopts the following technical solutions: a virtual synchronous control method for high-voltage direct current grid-connected island doubly-fed wind farms, which is characterized by comprising the following steps: 1) configuring the direct current busbars of back-to-back converters for DFIG wind turbines The DC power supply adopts the vector control method of constant AC voltage, and establishes the terminal voltage through the grid-side converter, wherein the terminal voltage is used to provide the initial power supply for the black start of the high-voltage DC system; 2) According to the established terminal voltage, VSG is applied to the grid-connected and normal operation control of DFIG wind turbines. After VSG realizes reactive voltage droop control through the rotor-side converter, the grid-side converter can be flexibly switched to unity power factor control. The DC power supply is withdrawn; 3) After the DC power supply is withdrawn, during the normal grid-connected operation of the wind turbine, when the VSG realizes the reactive power and voltage droop control, the motion equation of the synchronous machine rotor is simulated. There is a coupling relationship between them, and the damping characteristic makes the steady-state frequency deviation affect the output active power of the wind farm. The frequency adjustment and correction control method is used to achieve the frequency stability and active power balance of the sending end bus of the system through the rectifier, and the switching filter is used to correct the sending power. The frequency of the terminal bus is close to the rated value; 4) After the wind turbines equipped with DC power supply are started and connected to the grid, the remaining wind turbines without DC power supply are started.

Further, in the step 2), the specific steps of the unit power factor control process are: 2.1) after the standby terminal voltage is constant, close the rotor-side circuit breaker; the DC power supply provides rotor excitation for the startup of the DFIG through the rotor-side converter, based on The VSG pre-synchronization control realizes the no-load start-up of the DFIG unit, so that the stator voltage of the unit meets the grid connection condition; 2.2) After the stator voltage reaches the grid connection condition, the stator side circuit breaker is closed, the unit is connected to the grid, and the rotor side converter is switched to VSG normal 2.3) After the standby terminal voltage tends to be stable, the DC power supply should be withdrawn, the grid-side converter is switched to the traditional unit power factor control, and the reactive current is controlled to gradually become zero, so as to realize the unit power factor control .

Further, in the step 2.1), the method for making the stator voltage of the generator set meet the grid-connected conditions is: before the generator set is connected to the grid, the circuit breaker on the stator side is in a disconnected state, assuming that there is a virtual impedance R _v +sL _v between the two voltages, Then the generated virtual current i _v is:

Then calculate the virtual active and reactive power; when the virtual power is zero, the grid connection condition is satisfied; where R _v represents the virtual resistance, s represents the Laplace operator, and L _v represents the virtual inductance.

Further, in the step 2.2), the normal control process of the rotor-side converter VSG is as follows:

P _ref -P _s =Jdω _vsg /dt+D _p (ω _vsg -ω ₀ )

(K _p +K _i /s)] D _q (U _n -U _s )-Q _s [=E

θ _sr =∫(ω _vsg -ω _r )dt

In the formula, P _ref represents the maximum power tracking reference value, P _s represents the actual active power of the stator, J represents the virtual inertia, D _p represents the virtual damping coefficient, ω _vsg represents the virtual synchronous speed, ω ₀ represents the rated synchronous speed, K _p represents no The proportional parameter of the power regulator, K _i represents the integral parameter of the reactive power regulator, D _q represents the reactive voltage droop coefficient; U _n represents the rated voltage of the stator, U _s represents the actual voltage of the stator, Q _s represents the actual reactive power of the stator, and E represents the Rotor excitation voltage amplitude reference value, θ _sr represents the rotor excitation voltage phase angle reference value, ω _r represents the rotor speed, U _ref represents the final rotor excitation voltage reference value, R _v1 represents the virtual resistance for current limit control; I _rabc represents the rotor current.

Further, in the step 2.3), the switching method of the grid-side converter being switched to the traditional unity power factor control is: before switching, record the reactive current value of the grid-side converter at the end of the last control period; The side converter is switched to the traditional unity power factor control, and its reactive current command value is set to start ramping to zero from the last recorded current value; when the reactive current reaches zero, the grid-side converter realizes the traditional control. Unity power factor control.

Further, in the step 3), the system frequency adjustment and correction control methods are: frequency adjustment: when the active power of the DFIG unit starts to rise, the HVDC system is unlocked, the inverter adopts constant DC voltage control to establish a DC voltage, and the rectifier adopts frequency control to adjust Active power balance at the DC sending end: when the system frequency increases, the output of the wind farm increases, and the active power at the sending end is excessive. By adjusting the trigger angle, the DC current increases to realize the active power balance control of the sending end bus; frequency correction: the frequency of the sending end bus tends to increase. After the steady state, the system frequency is corrected by the method of switching filter to make it closer to the rated frequency, so that the DFIG is closer to the maximum power point tracking operation, and the method of switching filter is used to correct the system frequency to make it closer to the rated frequency.

Further, in the frequency correction, the method of switching the filter is adopted to correct the system frequency, and by switching the filter, the fundamental frequency capacitance of the filter of the sending-end bus is set as C _f , and the fundamental frequency of the filter of the sending-end bus is changed. The size of the capacitor C _f is realized, specifically: when the system frequency in the steady state is higher than the rated frequency, adding multiple filters to increase the value of C _f will reduce the system frequency; when the system frequency in the steady state is lower than the rated frequency , cut off multiple filters to reduce the C _f value, then the system frequency will increase.

A virtual synchronous control device for high-voltage DC grid-connected island doubly-fed wind farms, characterized in that the device includes a DC power supply configuration module, a system startup and grid-connected control module, a system frequency adjustment and correction control module, and the entire wind farm sequence start-up module; the DC power supply configuration module configures the DC power supply for the DC bus of the back-to-back converter of the DFIG wind turbine, adopts the vector control method of constant AC voltage, and establishes the machine terminal voltage through the grid-side converter, and the extreme voltage is used for The black start of the HVDC system provides the initial power supply; the system start-up and grid-connection control module applies the VSG to the pre-grid-connection and normal operation control of the DFIG wind turbine based on the established machine terminal voltage. After the rotor-side converter realizes reactive power and voltage droop control, the grid-side converter can be flexibly switched to unity power factor control, and the DC power supply is withdrawn at the same time; the system frequency adjustment and correction control module After the DC power supply is withdrawn, the wind turbine generator During normal grid-connected operation, when VSG realizes reactive power and voltage droop control, it simulates the equation of motion of the rotor of the synchronous machine. The inertia characteristic makes the coupling relationship between the active power of the wind farm and the frequency of the sending bus of the system, and the damping characteristic makes the steady-state frequency deviation to The output active power of the wind farm has an impact. The frequency adjustment and correction control methods are adopted. The rectifier is used to achieve the frequency stability and active power balance of the sending end bus of the system, and the switching filter is used to correct the sending end bus frequency close to the rated value; the entire wind farm starts in sequence. The module is to start the remaining wind turbines without DC power supply after the wind turbines configured with DC power supply are started and connected to the grid.

Further, the system frequency adjustment and correction control module includes a frequency adjustment module and a frequency correction module; the frequency adjustment module is that when the active power of the DFIG unit starts to rise, the HVDC system is unlocked, and the inverter adopts constant DC voltage control to establish a DC voltage. , the rectifier adopts frequency control to adjust the active power balance of the DC sending end: when the system frequency increases, the output of the wind farm increases, and the active power at the sending end is excessive. By adjusting the trigger angle, the DC current increases to realize the active power balance control of the sending end bus; The frequency correction module corrects the system frequency to be closer to the rated frequency by the method of switching filter after the bus frequency of the sending end tends to be stable, so that the DFIG is closer to the maximum power point tracking operation, and the switching filter is adopted. The method corrects the system frequency so that it is close to the rated frequency.

Further, the method of switching filter is adopted in the frequency correction module to correct the system frequency, and the fundamental frequency capacitance of the filter of the sending end bus is set to be C _f , and the size of the fundamental frequency capacitance C _f of the filter of the sending end bus is changed to realize , specifically: when the system frequency is higher than the rated frequency in the steady state, put in multiple filters to increase the C _f value, then the system frequency will decrease; when the system frequency in the steady state is lower than the rated frequency, cut off multiple filters Decreasing the value of C _f will increase the system frequency.

The present invention has the following advantages due to the adoption of the above technical solutions: 1. The present invention adopts LCC-HVDC to realize the high-voltage direct current grid connection of the islanded wind farm. Compared with the VSC-HVDC solution, the system has higher capacity, lower power loss and construction cost. 2. The present invention adopts the DC power supply for the DC bus of the DFIG unit, which can provide necessary conditions for the startup of the system. In fact, the current wind farm units are generally equipped with a DC power supply for pitch angle action and the like. 3. The present invention adopts the VSG technology to make the wind turbines realize the drooping characteristic of reactive power and voltage, so the wind turbines can operate in parallel. 4. The present invention adopts VSG technology to simulate the motion equation of the synchronous machine, has certain inertia and damping characteristics, and is beneficial to increase the stability of the system. 5. The present invention uses the switching filter to correct the system frequency, reduces the influence of the VSG damping term on the maximum power tracking during the steady state, and improves the active benefit of the wind farm.

Description of drawings

Fig. 1 is a topological schematic diagram of an island DFIG type wind farm connected to a high-voltage direct current system in the present invention, RSC represents a rotor-side converter, GSC represents a grid-side converter, and SEB represents a DC sending end bus;

Figure 2 is an equivalent circuit diagram of a single-machine equivalent wind farm connected to a HVDC system;

Fig. 3 is the relative relationship of each vector under the control of VSG, in the figure RSRF represents the reference coordinate line rotating at the rated synchronous speed, SSRF represents the reference coordinate system rotating at the system frequency, and VSRF represents the reference coordinate system rotating at the virtual synchronous speed;

Figure 4 system control block diagram, including RSC and GSC control of DFIG unit, rectifier and inverter control of HVDC system;

Figure 5(a) Schematic diagram of the simulation results of the bus voltage at the sending end and the terminal voltage of the DFIG unit;

Figure 5(b) Schematic diagram of the simulation results of terminal voltage and stator voltage of the first batch of equivalent units;

Figure 5(c) Schematic diagram of the simulation results of stator active power command value and actual value;

Figure 5(d) Schematic diagram of simulation results of system frequency and virtual synchronization frequency;

Figure 5(e) Schematic diagram of the simulation results of the command value and actual value of the DC bus voltage of the unit converter;

Figure 5(f) Schematic diagram of the simulation results of the command value and actual value of the reactive current of the grid-side converter of the unit;

Figure 5(g) Schematic diagram of the simulation results of stator voltage and terminal voltage of the second batch of equivalent units;

Fig. 5(h) Schematic diagram of the simulation result of stator active power command value and actual value;

Fig. 5(i) Schematic diagram of simulation result of rectifier firing angle command value;

Figure 5(j) Schematic diagram of the simulation results of the DC current command value and actual value of the DC system;

Figure 5(k) Schematic diagram of the simulation results of the DC system voltage and current;

Fig. 5(l) The schematic diagram of the active power simulation results of the wind turbine and the DC system;

Figure 5(m) The schematic diagram of the reactive power simulation results of the wind turbine, filter and DC system.

Detailed ways

The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

The present invention provides a virtual synchronization control method for high-voltage direct current grid connection of an island doubly-fed wind farm, which comprises the following steps:

1) Configure DC power supply

Select the DFIG wind turbine that meets the minimum power requirements of the HVDC system with the sum of the active power, configure the DC power supply for the DC bus of the back-to-back converter, and use the vector control method of constant AC voltage to provide a starting point for the black start of the HVDC system. powered by.

1.1) Configure the DC power supply for the back-to-back DC bus of the DFIG wind turbine. The DC power supply can play two roles: firstly, to establish the machine terminal voltage through the grid-side converter, and secondly, through the rotor-side converter for DFIG Start provides rotor excitation.

1.2) As shown in Figure 1, after the system receives the start command, it closes the circuit breaker at the DC power supply, and the DC power supply charges the DC bus capacitor until it reaches the rated voltage. In order to generate a constant terminal voltage, a constant AC voltage is used. The vector control method generates a constant terminal voltage through the grid-side converter of the unit. The specific process is as follows:

The mathematical model of the grid-side converter can be expressed as

C _dc du _dc /dt=i _{dc_r} -(m _gcd i _gcd +m _gcq i _gcq ) (2)

In the formula, m _gcd and m _gcq represent the modulation ratio of the d-axis and q-axis voltages, respectively, L _gc represents the grid-connected inductance on the GSC side, R _gc represents the internal resistance of the grid-connected inductance on the GSC side, and u _dc represents the DC bus voltage of the converter , ω _sys represents the system frequency, i _gcd represents the d-axis current on the GSC side, i _gcq represents the q-axis voltage on the GSC side, u _sd represents the d-axis stator voltage, and u _sq represents the q-axis stator voltage, as shown in Figure 2, the abc has been The variables and parameters in the coordinate system are transformed into the dq coordinate system.

In order to achieve constant AC voltage control, the specified reference coordinate system rotates at the rated frequency, as shown in Figure 4, by controlling u _sd and u _sq to be 1 and 0 respectively, the control goal of constant AC voltage amplitude and frequency can be achieved.

2) System startup and grid-connected control

The VSG (virtual synchronous machine method) is applied to the control of pre-grid connection and normal operation after grid connection of DFIG wind turbines to realize the simulation of the operation characteristics of wind turbines connected to LCC-HVDC to traditional synchronous wind turbines connected to LCC-HVDC. After the VSG realizes the reactive voltage droop control through the rotor-side converter, the grid-side converter can be flexibly switched to unity power factor control, and the DC power supply should be withdrawn at the same time.

2.1) According to the generated machine terminal voltage, after the standby terminal voltage is constant, close the rotor side circuit breaker. The DC power supply provides rotor excitation for the startup of the DFIG through the rotor-side converter, and realizes the no-load startup of the DFIG unit based on the VSG pre-synchronization control, so that the stator voltage of the unit meets the grid-connected conditions. The specific process is as follows:

Before the unit is connected to the grid, the circuit breaker on the stator side is in the disconnected state. In order to realize the grid connection condition of the same amplitude, frequency and phase of the stator voltage u _s and the generator terminal voltage u _t , it is assumed that there is a virtual impedance R _v + between the two voltages. sL _v (R _v represents virtual resistance, s represents Laplace operator, and L _v represents virtual inductance), then the generated virtual current i _v is:

Based on this, virtual active and reactive power can be calculated. Obviously, when the virtual power is zero, the grid-connected condition is satisfied. As shown in FIG. 4, the VSG pre-synchronization control can gradually adjust the virtual power to zero.

2.2) After the stator voltage reaches the grid connection condition, close the circuit breaker on the stator side, and the unit is connected to the grid. At this time, the rotor-side converter is switched to VSG normal control (as shown in Figure 4), and the active power gradually increases. The process of VSG control is as follows:

P _ref -P _s =Jdω _vsg /dt+D _p (ω _vsg -ω ₀ ) (4)

(K _p +K _i /s)[D _q (U _n -U _s )-Q _s ]=E (5)

θ _sr =∫(ω _vsg -ω _r )dt (6)

In the formula, P _ref represents the maximum power tracking reference value, P _s represents the actual active power of the stator, J represents the virtual inertia, D _p represents the virtual damping coefficient, ω _vsg represents the virtual synchronous speed, ω ₀ represents the rated synchronous speed, K _p represents no The proportional parameter of the power regulator, K _i represents the integral parameter of the reactive power regulator, D _q represents the reactive voltage droop coefficient; U _n represents the rated voltage of the stator, U _s represents the actual voltage of the stator, Q _s represents the actual reactive power of the stator, and E represents the Rotor excitation voltage amplitude reference value, θ _sr represents the rotor excitation voltage phase angle reference value, ω _r represents the rotor speed, U _ref represents the final rotor excitation voltage reference value, R _v1 represents the virtual resistance for current limit control; I _rabc represents the rotor current. The relative relationship between the rotor-side and stator-side vectors is shown in Figure 3.

2.3) The rotor-side converter controls the active power rise and realizes the reactive power voltage droop control, and participates in the voltage regulation of the machine terminal. After the standby terminal voltage tends to be stable, the DC power supply should be withdrawn. After that, the grid-side converter should no longer maintain constant AC voltage control, but should switch to traditional unity power factor control, control the reactive current to gradually become zero, and achieve unity power factor control.

However, when the DC power supply is withdrawn, the reactive current of the grid-side converter may not be zero. If it is directly switched to unity power factor control, there will be a large command step, which may cause a large overshoot process or even failure. stable. In order to achieve flexible switching, the following switching methods are used:

Before switching, record the reactive current value of the grid-side converter at the end of the last control cycle; the grid-side converter is switched to traditional unity power factor control, and set its reactive current command value to the recorded current at the last moment The value starts to ramp to zero, that is, the control reactive current gradually becomes zero instead of a direct step to zero; when the reactive current reaches zero, the grid-side converter implements traditional unity power factor control.

3) System frequency adjustment and correction control

After the DC power supply is withdrawn, during the normal grid-connected operation of the wind turbine, when the VSG realizes the reactive voltage droop control, the motion equation of the synchronous machine rotor is simulated. The damping characteristic makes the steady-state frequency deviation affect the output active power of the wind farm. Therefore, the frequency regulation and correction control method is adopted, and the frequency stability and active power balance of the sending end bus of the system can be achieved through the rectifier.

Frequency regulation: When the active power of the DFIG unit starts to rise, the HVDC system is unlocked, the inverter uses constant DC voltage control to establish the DC voltage, and the rectifier uses frequency control to adjust the active power balance of the DC sending end: when the system frequency increases, it means that the output of the wind farm increases. If it is large, the active power of the sending end is excessive. At this time, the DC current is increased by adjusting the trigger angle, and the active power balance control of the sending end bus is realized. Active power balance control is shown in Figure 4. It is worth noting that the inverter still adopts the traditional constant voltage control method.

Frequency correction: After the bus frequency at the sending end tends to be stable, the system frequency is corrected by switching filters to make it closer to the rated frequency, so that the DFIG is closer to the maximum power point tracking operation: after the wind farm is started, due to the DC transmission There is no constant voltage source at the terminals to clamp the system frequency, so the system frequency may deviate from the rated frequency. It can be known from the VSG control that the damping term D _p (ω _vsg -ω ₀ ) has a great influence on the MPPT (maximum power point tracking) control at this time. In order to correct the system frequency to make it as close to the rated frequency as possible, the method of switching filter can be used, and the specific process is as follows:

Assuming that the fundamental frequency capacitance of the filter of the sending-end bus is C _f , then in the rated frequency rotating reference coordinate system, the mathematical model of the sending-end bus can be expressed as:

In the formula, u _ibd represents the d-axis voltage of the sending bus, i _wd represents the d-axis current of the wind farm port, i _rcd represents the d-axis current of the rectifier, u _ibq represents the q-axis voltage of the sending bus, and i _wq represents the q-axis current of the wind farm port , i _rcq represents the rectifier q-axis current;

Transforming formula (8) into the polar coordinate system, and further sorting out:

In formula (9), P _w = u _ibd i _wd + u _ibq i _wq is the active power emitted by the wind farm, P _rc = u _ibd i _rcd + u _ibq i _rcq is the active power absorbed by the rectifier, and U _ibm is the sending bus voltage Amplitude, φ represents the phase angle of the bus voltage at the sending end. In formula (10), Q _w =-u _ibd i _wq +u _ibq i _wd is the reactive power emitted by the wind farm, and Q _rc =-u _ibd i _rcq +u _ibq i _rcd is the reactive power absorbed by the rectifier.

It can be seen from this that the sending-end bus voltage amplitude U _ibm is strongly related to the active power balance, while the sending-end bus frequency ω _sys is related to both reactive power balance and voltage amplitude. Therefore, ω _sys is related to both the active power balance and the reactive power balance. It can be seen from this that, as shown in FIG. 3 , the virtual synchronous rotational speed ω _vsg is related to both the active power balance and the reactive power balance. In order to correct the system frequency, it can be realized by switching the filter, that is, changing the size of C _f . The details are as follows: when the system frequency is higher than the rated frequency in the steady state, input multiple filters to increase the value of C _f , and it can be seen from equation (10) that the system frequency decreases; when the system frequency in the steady state is lower than the rated frequency, remove the Multiple filters reduce the value of _Cf , whereby the system frequency will increase.

Take the system frequency drop as an example to illustrate the change of the active power of the wind farm. When the frequency drops, it can be seen from the VSG control block diagram that the active power of the stator of the wind turbine will increase, and the virtual synchronous frequency will also decrease. After reaching a new steady state, the excess kinetic energy stored in the rotor is released due to the damping term, and the active power emitted by the wind farm will increase, approaching MPPT operation.

4) Start-up sequence of the entire wind farm

In order to reduce the cost of DC power configuration, there is no need to configure DC power for all units in the wind farm. When the units equipped with the DC power supply are started, the remaining units can be started with the help of the electric energy generated by them, and the virtual synchronous control of the high-voltage DC grid-connected island DFIG wind farm can be realized. Therefore, the start-up of the entire wind farm adopts a batch-by-batch start-up method.

4.1) The first batch of wind turbines to be started are those equipped with DC power. In order to ensure the reliability of starting, these units must be started at the same time, so that the terminal voltages of each unit are synchronized.

4.2) The second batch of units (that is, the remaining units) are started in succession, and the DC voltage of the unit converters can be established directly by the grid-side converters absorbing the power generated by the units that have already been started, that is, there is no need to configure a DC power supply. On the one hand, during the start-up process of the second batch of units, the VSG pre-synchronization control can realize the synchronization of the stator voltage and the terminal voltage, and each unit does not need to be started at the same time. On the other hand, in order to reduce the impact and disturbance on the units already started, the remaining units should be started and connected to the grid one after another.

Example:

The LCC-HVDC system with a rated capacity of 1000MW, the parameters are derived from the CIGRE standard model. The DFIG wind farm consists of two single machines equivalent. One of them represents the first batch of units to be started, with a rated capacity of 200×1.5MVA=300MVA, and the other represents the second batch of units to be started, with a rated capacity of 433×1.5MVA=500MVA. Inventions are exemplified in this system.

1) DC power configuration and terminal voltage generation

1.1) Configure the DC power supply for the back-to-back DC bus of the DFIG wind turbine, as shown in Figure 4.

1.2) The system receives the start command and closes the circuit breaker s ₄ at the DC bus in Figure 4. After the DC voltage is charged to the rated value, the switches s ₅ , s ₆ and s ₇ in the GSC control block diagram are all thrown to 2 at 0.1s , the vector control method of constant AC voltage is used to generate a constant AC terminal voltage, as shown in Figure 5(a).

2) System startup and grid-connected control

2.1) After the voltage at the standby terminal is stable, start the RSC pre-synchronization control at 0.2s, and throw the switches s ₁ , s ₂ and s ₃ in Fig. 4 to 2, and the DC power supply provides rotor excitation for the startup of DFIG through RSC, based on VSG The pre-synchronization control realizes the no-load start-up of the DFIG unit, as shown in Fig. 5(b).

2.2) After the stator voltage reaches the grid-connected condition, close the circuit breaker on the stator side. At 0.7s, the switches s ₁ , s ₂ and s ₃ in Figure 4 are all thrown to 1. At this time, the RSC is switched to the normal control of the VSG, and the active power begins to gradually increase, as shown in Figure 5(c)-Figure 5(d). Show.

2.3) After the voltage at the machine terminal enters a steady state, the DC power supply is withdrawn, and the circuit breaker s ₄ in Fig. 4 is disconnected. At 0.8s, the switches s ₅ , s ₆ and s ₇ in the GSC control block diagram are all thrown to 1, and the reactive current is controlled to gradually become zero. After that, the traditional vector control based on PLL-oriented unity power factor is adopted, as shown in Figure 5 (e ) - shown in Figure 5(f).

3) HVDC system startup, system frequency adjustment and correction control

3.1) When the active power of the DFIG unit starts to rise, the HVDC system is unlocked, and the switch s ₈ in Figure 4 is switched to 1 at 0.7s. The inverter adopts constant DC voltage control to establish the DC voltage, and the rectifier adopts frequency control to adjust the active power balance of the DC sending end, as shown in Figure 5(i)-Figure 5(k).

3.2) After the bus frequency at the sending end tends to be stable, the system frequency still deviates from the rated value at about 3.6s, as shown in Figure 5(d). In Fig. 5(i)-Fig. 5(j), the firing angle has reached the limit value, and the DC current is no longer rising. 3.8s, put in a 200Mvar filter, as shown in Figure 5(m). After that, the system frequency dropped rapidly, as shown in Fig. 5(d), and the active power of the wind farm continued to rise, as shown in Fig. 5(c) and Fig. 5(h), tending to the MPPT command value.

4) Start-up sequence of the entire wind farm

4.1) The first batch of wind turbines to start is as described above.

4.2) At 2.0s, the second batch of equivalent units starts up. First, the DC voltage of the converter is established through GSC, and then pre-synchronization is achieved through RSC, as shown in Figure 5(g). At 2.6s, the unit was connected to the grid, and the active power began to rise, as shown in Figure 5(h) and Figure 5(l).

In addition, at about 6s, the wind speed changes from 11m/s to 6m/s. It can be seen from Figure 5 that the DC system can track the change of the active power of the wind farm, and the system still maintains stable operation, which shows the feasibility of the rectifier frequency and active power balance control. sex. It is worth noting that it takes longer for the system to converge after the wind speed changes in Figure 5, which is not shown in the figure due to the limited space.

The present invention also provides a virtual synchronous control device for high-voltage DC grid-connected island doubly-fed wind farms, which includes a DC power supply configuration module, a system startup and grid-connected control module, a system frequency adjustment and correction control module, and a sequence startup module for the entire wind farm ;

The DC power supply configuration module configures the DC power supply for the DC bus of the back-to-back converter of the DFIG wind turbine, adopts the vector control method of constant AC voltage, and establishes the machine terminal voltage through the grid-side converter. Start to provide initial power supply;

The system startup and grid-connection control module applies the VSG to the pre-grid-connection and normal operation control of the DFIG wind turbine based on the generated terminal voltage. After the VSG realizes the reactive voltage droop control through the rotor-side converter, The grid-side converter can be flexibly switched to unity power factor control, and the DC power supply is withdrawn at the same time;

The system frequency adjustment and correction control module simulates the rotor motion equation of the synchronous machine when the VSG realizes the reactive power and voltage droop control during the normal grid-connected operation of the wind turbine after the DC power supply is withdrawn. There is a coupling relationship between the bus frequencies, and the damping characteristic makes the steady-state frequency deviation affect the active power output of the wind farm. The frequency adjustment and correction control methods are adopted to achieve the frequency stability and active power balance of the sending end bus of the system through the rectifier. Correct the sending end bus frequency close to the rated value;

The entire wind farm sequence startup module is to start the remaining wind turbines without DC power supply after the wind turbines equipped with DC power supply are started and connected to the grid.

In the above-mentioned embodiment, the system startup and grid-connection control module further includes a generator set stator voltage grid-connection module, a rotor-side converter VSG normal control module, and a unity power factor control switching module.

The stator voltage grid-connected module of the unit closes the rotor-side circuit breaker after the voltage at the standby terminal is constant; the DC power supply provides rotor excitation for the startup of the DFIG through the rotor-side converter, and the no-load startup of the DFIG unit is realized based on the VSG pre-synchronization control, so that the The stator voltage of the unit meets the grid connection conditions;

The rotor-side converter VSG normal control module is to close the stator-side circuit breaker after the stator voltage reaches the grid-connected condition, the unit is connected to the grid, the rotor-side converter is switched to VSG normal control, and the active power gradually increases;

When the voltage at the standby terminal of the unit power factor control switching module tends to be stable, the DC power supply should be withdrawn, and the grid-side converter is switched to the traditional unit power factor control, which controls the reactive current to gradually become zero, and realizes the unit power factor control.

In the above embodiment, before the generator set stator voltage grid-connected module is connected to the grid, the circuit breaker on the stator side is in an off state. Assuming that there is a virtual impedance R _v +sL _v between the two voltages, the generated virtual current _iv is:

Then calculate the virtual active and reactive power; when the virtual power is zero, the grid connection condition is satisfied; where R _v represents the virtual resistance, s represents the Laplace operator, and L _v represents the virtual inductance.

In the above embodiment, the control process of the rotor-side converter VSG normal control module is as follows:

P _ref -P _s =Jdω _vsg /dt+D _p (ω _vsg -ω ₀ )

(K _p +K _i /s)[D _q (U _n -U _s )-Q _s ]=E

θ _sr =∫(ω _vsg -ω _r )dt

In the above embodiment, the switching method of the unit power factor control switching module is: before switching, record the reactive current value of the grid-side converter at the end of the last control period; the grid-side converter is switched to the traditional unit power factor. Control, set its reactive current command value to start the ramp change to zero at the last moment of recording; when the reactive current reaches zero, the grid-side converter realizes the traditional unity power factor control.

In the above embodiments, the system frequency adjustment and correction control module includes a frequency adjustment module and a frequency correction module. The frequency adjustment module is that when the active power of the DFIG unit starts to rise, the HVDC system is unlocked, the inverter uses constant DC voltage control to establish the DC voltage, and the rectifier uses frequency control to adjust the active power balance of the DC sending end: when the system frequency increases, the output of the wind farm increases. If it is large, the active power of the sending end is excessive. By adjusting the trigger angle, the DC current is increased, and the active power balance control of the sending end bus is realized. The frequency correction module corrects the system frequency to be closer to the rated frequency by the method of switching filter after the bus frequency of the sending end tends to be stable, so that the DFIG is closer to the maximum power point tracking operation, and the switching filter is adopted. The method corrects the system frequency so that it is close to the rated frequency.

In the above-mentioned embodiments, the method of switching filter is adopted in the frequency correction module to correct the system frequency, and the fundamental frequency capacitance of the filter of the sending end bus is set to be C _f , and the fundamental frequency capacitance C _f of the filter of the sending end bus is changed. The size is realized, specifically: when the system frequency is higher than the rated frequency in the steady state, put in multiple filters to increase the C _f value, then the system frequency will decrease; when the system frequency in the steady state is lower than the rated frequency, cut off multiple filters The filter reduces the C _f value, the system frequency will increase.

The above embodiments are only used to illustrate the present invention, wherein the system capacity, start-up time, etc. will vary according to the specific parameters of the DC project. Those skilled in the art should understand that the embodiments of the present application can be provided as methods and systems. or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) having computer-usable program code embodied therein.

Claims (10)

1. A virtual synchronization control method for high-voltage direct current grid connection of an island doubly-fed wind power plant is characterized by comprising the following steps:

1) configuring a direct-current power supply for a direct-current bus of a back-to-back converter of a DFIG wind turbine generator system, and establishing a generator terminal voltage through a grid-side converter by adopting a vector control method of a fixed alternating-current voltage, wherein the generator terminal voltage is used for providing initial power supply for black start of a high-voltage direct-current system;

2) according to the established generator terminal voltage, VSG is applied to pre-grid connection of the DFIG wind turbine generator and normal operation control after grid connection, after the VSG realizes reactive voltage droop control through a rotor side converter, the grid side converter is flexibly switched to unit power factor control, and a direct-current power supply exits;

3) after the direct-current power supply is withdrawn, when VSG realizes reactive voltage droop control during the normal grid-connected operation of a wind turbine generator, a synchronous machine rotor motion equation is simulated, wherein the inertia characteristic enables the active power of a wind power plant and the frequency of a system sending end bus to have a coupling relation, the damping characteristic enables steady-state frequency deviation to influence the output active power of the wind power plant, a frequency regulation and correction control method is adopted, the frequency stability and the active balance of the system sending end bus are realized through a rectifier, and the frequency of the sending end bus is corrected to be close to a rated value through a switching filter;

4) and after the wind turbine generator set configured with the direct-current power supply is started and the grid connection is completed, starting the rest wind turbine generator sets not configured with the direct-current power supply.

2. The virtual synchronization control method according to claim 1, characterized in that: in the step 2), the specific steps of the unit power factor control process are as follows:

2.1) closing the rotor side circuit breaker after the standby terminal voltage is constant; the direct-current power supply provides rotor excitation for starting the DFIG through a rotor-side converter, and the no-load starting of the DFIG unit is realized based on VSG pre-synchronization control, so that the stator voltage of the unit meets grid-connected conditions;

2.2) after the stator voltage reaches the grid-connected condition, closing a stator side breaker, connecting the unit with the grid, switching a rotor side converter into VSG (voltage source generator) for normal control, and gradually increasing the active power;

2.3) after the standby terminal voltage tends to be stable, the direct current power supply should exit, the grid-side converter is switched to the traditional unit power factor control, the reactive current is controlled to gradually become zero, and the unit power factor control is realized.

3. The virtual synchronization control method according to claim 2, characterized in that: in the step 2.1), the method for enabling the stator voltage of the unit to meet the grid-connected condition comprises the following steps: before the unit is connected to the grid, the breaker on the stator side is in an off state, and a virtual impedance R exists between two voltages_v+sL_vA virtual current i is generated_vComprises the following steps:

virtual active and reactive power are calculated; when the virtual power is zero, the grid-connected condition is met; wherein R is_vRepresenting a virtual resistance, s representing the Laplace operator, L_vRepresenting a virtual inductance u_sRepresenting the stator voltage u_tRepresenting the terminal voltage.

4. The virtual synchronization control method according to claim 2, characterized in that: in the step 2.2), the process of normally controlling the rotor-side converter VSG is as follows:

P_ref-P_s＝Jdω_vsg/dt+D_p(ω_vsg-ω₀)

(K_p+K_i/s)[D_q(U_n-U_s)-Q_s]＝E

θ_sr＝∫(ω_vsg-ω_r)dt

in the formula, P_refRepresenting the maximum power tracking reference value, P_sRepresenting the actual active power of the stator, J representing the virtual inertia, D_pRepresenting a virtual damping coefficient, ω_vsgRepresenting virtual synchronous speed, ω₀Indicating nominal synchronous speed, K_pRepresenting a proportional parameter, K, of the reactive regulator_iRepresenting integral parameters of the reactive regulator, D_qRepresenting a reactive voltage droop coefficient; u shape_nIndicating stator rated voltage, U_sRepresenting the actual stator voltage, Q_sRepresenting the actual reactive power of the stator, E representing the reference value of the amplitude of the rotor excitation voltage, theta_srRepresenting the phase angle reference value, omega, of the rotor excitation voltage_rIndicating rotor speed, U_refRepresenting the final resulting rotor excitation voltage reference value, R_v1Representing a virtual resistance for current limit control; i is_rabcRepresenting the rotor current.

5. The virtual synchronization control method according to claim 2, characterized in that: in the step 2.3), a switching method for switching the grid-side converter to the conventional unit power factor control includes: before switching, recording the reactive current value of the grid-side converter at the end moment of the last control period; the grid-side converter is switched to the traditional unit power factor control, and the reactive current instruction value of the grid-side converter is set to start slope change to be zero by the recorded current value at the last moment; when the reactive current reaches zero, the grid-side converter realizes the traditional unit power factor control.

6. The virtual synchronization control method according to claim 1, characterized in that: in the step 3), the system frequency adjustment and correction control methods respectively comprise:

frequency adjustment: when the active power of the DFIG unit begins to rise, the HVDC system is unlocked, the inverter adopts fixed direct current voltage to control and establish direct current voltage, and the rectifier adopts frequency control to adjust the active balance of a direct current sending end: when the system frequency is increased, the output of the wind power plant is increased, the active power of a sending end is surplus, and the direct current is increased by adjusting a trigger angle, so that the active power balance control of a sending end bus is realized;

and (3) frequency correction: after the frequency of the transmission end bus tends to be stable, the system frequency is corrected by a method of switching a filter to be closer to the rated frequency, so that the DFIG is closer to the maximum power point tracking operation, and the system frequency is corrected by a method of switching the filter to be closer to the rated frequency.

7. The virtual synchronization control method according to claim 6, wherein: in the frequency correction, the system frequency is corrected by adopting a method of switching a filter, and the fundamental frequency capacitance of the filter of a transmission end bus is set as C_fChanging the fundamental frequency capacitance C of the filter of the transmitting end bus_fThe size of the method is realized by the following steps: when the system frequency is higher than the rated frequency in the steady state, a plurality of filters are put into use to increase C_fValue, then the system frequency decreases; cutting off multiple filters reduces C when the system frequency is below the nominal frequency at steady state_fValue, the system frequency will increase.

8. The utility model provides a virtual synchronization control device that island double-fed wind-powered electricity generation field high voltage direct current is incorporated into power networks which characterized in that: the device comprises a direct-current power supply configuration module, a system starting and grid-connected control module, a system frequency adjusting and correcting control module and a whole wind power plant time sequence starting module;

the direct-current power supply configuration module is used for configuring a direct-current power supply for a direct-current bus of a back-to-back converter of the DFIG wind turbine generator system, a vector control method of constant alternating-current voltage is adopted, a generator terminal voltage is established through a network side converter, and the generator terminal voltage is used for providing initial power supply for black start of a high-voltage direct-current system;

the system starting and grid-connected control module applies VSG to pre-grid connection of the DFIG wind turbine generator and normal operation control after grid connection according to the established generator terminal voltage, after the VSG realizes reactive voltage droop control through a rotor-side converter, the grid-side converter is flexibly switched to unit power factor control, and the direct-current power supply exits;

the system frequency adjusting and correcting control module simulates a synchronous machine rotor motion equation when VSG realizes reactive voltage droop control during the normal grid-connected operation period of a wind turbine generator after a direct-current power supply is withdrawn, wherein inertia characteristics enable coupling relation to exist between active power of a wind power plant and system sending end bus frequency, damping characteristics enable steady-state frequency deviation to affect output active power of the wind power plant, a frequency adjusting and correcting control method is adopted, system sending end bus frequency stabilization and active balance are realized through a rectifier, and sending end bus frequency is corrected to be close to a rated value through a switching filter;

the whole wind power plant time sequence starting module starts the rest units which are not provided with the direct-current power supply after the wind turbine units provided with the direct-current power supply are started and the grid connection is completed.

9. The virtual synchronization control apparatus of claim 8, wherein: the system frequency adjusting and correcting control module comprises a frequency adjusting module and a frequency correcting module;

the frequency regulation module is used for unlocking the HVDC system when the active power of the DFIG unit begins to rise, the inverter adopts fixed direct current voltage for controlling and establishing direct current voltage, and the rectifier adopts frequency for controlling and regulating the active balance of a direct current sending end: when the system frequency is increased, the output of the wind power plant is increased, the active power of a sending end is surplus, and the direct current is increased by adjusting a trigger angle, so that the active power balance control of a sending end bus is realized;

the frequency correction module corrects the system frequency to be closer to the rated frequency by a method of switching a filter after the frequency of the transmission end bus tends to be stable, so that the DFIG is closer to the maximum power point tracking operation, and corrects the system frequency to be closer to the rated frequency by a method of switching the filter.

10. The virtual synchronization control apparatus of claim 9, wherein: the frequency correction module corrects the system frequency by adopting a method of switching a filter, and the fundamental frequency capacitance of the filter of the transmission end bus is set as C_fChanging the fundamental frequency capacitance C of the filter of the transmitting end bus_fThe size of the method is realized by the following steps: when the system frequency is higher than the rated frequency in the steady state, a plurality of filters are put into use to increase C_fValue, then the system frequency decreases; cutting off multiple filters reduces C when the system frequency is below the nominal frequency at steady state_fValue, the system frequency will increase.

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